Elucidation of spin reorientation and short-range ferromagnetic correlations in double perovskite Ho₂FeCoO₆

We report on energy resolved neutron diffraction and inelastic neutron scattering studies of double perovskite Ho₂FeCoO₆. The B-site moments have been shown previously to order antiferromagnetically near 250 K with a spin reorientation at 45 K. We use energy resolved neutron diffraction of a Ho₂FeCoO₆ powder to establish the order parameter of these two phases and to characterize a diffuse scattering signal observed surrounding q = 1.3 Å^-1. The diffuse part is determined to result from short-range ferromagnetic correlations of the Ho-moments. The development of Ho correlations coincides in temperature with the spin reorientation transition so it is likely that exchange between sublattices is becoming important in this temperature range. We have also explored this system via inelastic scattering and report on a crystalline electric field that is detected at E = 9.8 meV. Both the diffuse and inelastic signals were found to be absent in non-A-site magnetic La₂FeCoO₆ confirming the origin of these effects as the Ho sublattice.